The invention relates to densification of porous annular substrates by chemical vapor infiltration (CVI).
A particular field of application of the invention is the making of annular parts in a thermostructural composite material, such as carbon/carbon (C/C) composite brake discs for airplanes or land vehicles.
Thermostructural composite materials are remarkable because they possess mechanical properties that enable them to be used for making structural parts and have the ability to conserve these properties at high temperatures. Typical examples of thermostructural composite materials are C/C composite materials having a reinforcing fibrous texture of carbon fibers densified by a pyrolytic carbon matrix, and ceramic matrix composites (CMCs) having a reinforcing texture of refractory fibers (carbon or ceramic) densified by a ceramic matrix.
In a CVI process, substrates to be densified are placed in a reaction chamber of a furnace in which they are heated. A reactive gas containing one or more gaseous precursors of the material that is to constitute the matrix is introduced into the reaction chamber. The temperature and pressure inside the reaction chamber are adjusted to enable the reactive gas to diffuse within the pores of the substrate and deposit the matrix-constituting material therein by one or more components of the reactive gas decomposing or reacting together. The process is performed under low pressure in order to enhance diffusion of the reactive gas into the substrates. The temperature at which the precursor(s) is transformed to form the matrix material, such as pyrolytic carbon or ceramic, is usually greater than 900xc2x0 C., and is typically close to 1000xc2x0 C.
In order to enable substrates throughout the reaction chamber to be densified as uniformly as possible, whether in terms of increasing density or in terms of microstructure of the matrix material deposited, it would ideally be necessary to have a substantially uniform temperature within the reaction chamber and to allow the reactive gas to reach all substrates relatively uniformly.
CVI furnaces usually include a gas preheater situated inside the furnace between the reactive gas inlet into the furnace and the reaction chamber. Typically, a gas preheater zone comprises a heat exchange assembly in the form of a plurality of perforated plates through which the reactive gas passes before entering the reaction chamber.
The substrates, like the heat-exchange assembly, are heated because they are located in the furnace. The latter is generally heated by means of a susceptor, e.g. made of graphite. The susceptor defines the side of the wall of the reaction chamber and is heated by inductive coupling with an inductor surrounding the reaction chamber or by resistors surrounding the furnace.
The applicants have found that the efficiency of the gas preheater is not always as good as desired. A significant example is that of densifying porous substrates constituted by annular preforms of carbon fibers or pre-densified annular blanks for use in making C/C composite brake disks.
The annular substrates are loaded in vertical stacks in the reaction chamber above the gas preheater which is situated at the bottom of the furnace. In spite of the reactive gas being preheated, a temperature gradient is often observed in the reaction chamber, with the temperature close to substrates situated at the bottom of the stacks possible being several tens of xc2x0 C. lower than the temperature that applies in the remainder of the stacks. This may give rise to a large densification gradient between the substrates in a same stack, depending on the position of a substrate within the stack.
In order to solve that problem, it would be possible to increase the efficiency with which the reactive gas is preheated by increasing the size of the gas preheater. However, for a given volume of the furnace, that would reduce the loading capacity for the substrates. Since CVI processes require large amounts of industrial investment and long processing time, it is highly desirable for furnaces to have the highest possible productivity, and thus as high as possible a ratio of volume dedicated to the load of substrates over the volume dedicated to preheating the reactive gas.
Another problem resides in the fact that a temperature gradient is observed not only in the vertical direction, along the stacks of substrates, but also in the horizontal direction, between different stacks. In particular, it has been noted that stacks located in a central part of the reaction chamber may not benefit from the heat radiated by the susceptor in the same way as stacks located closer to the internal side wall of the susceptor.
This also results in a gradient of densification between substrates belonging to different stacks.
An object of the invention is generally to provide means for achieving an efficient and cost effective substantially uniform densification of porous annular substrates in a CVI furnace.
A particular object of the invention is to provide a gas preheater which allows such a substantially uniform densification to be achieved without significantly affecting the productivity of the CVI substrate.
According to one aspect of the invention, in a CVI furnace for the densification of annular porous substrates arranged in a plurality of vertical annular stacks of substrates, comprising a susceptor having an internal side wall delimiting a gas preheating zone and a reaction chamber within the furnace and a bottom wall, and at least one gas inlet opening through the bottom wall of the susceptor, a gas preheater is provided which comprises:
a sleeve made of heat conductive material resting upon the susceptor bottom wall and delimiting a gas preheating chamber, with the at least one gas inlet opening in the gas preheating chamber,
a heat exchange assembly located in the gas preheating chamber,
a gas distribution plate resting upon the sleeve, covering the gas preheating chamber and provided with a plurality of passages for pre-heated gas,
a load supporting plate for supporting stacks of annular substrates to be loaded in the reaction chamber for densification and provided with a plurality of passages in communication with respective passages of the gas distribution plate and in registration with internal volumes of respective stacks of annular substrates, and
nozzles inserted in passages communicating the gas preheating zone with the internal volumes of respective stacks of annular substrates for adjusting the flows of preheated gas respectively admitted in said internal volumes.
The sleeve, which is preferably formed of a massive body made in one piece of heat conductive material, achieves different functions:
resting upon the susceptor bottom wall and being thus surrounded by the susceptor side wall, it enables an efficient heating of the preheating zone to be reached,
it encloses the preheating zone and contributes to the sealing thereof, avoiding a large fraction of the reactive gas admitted to reach the reaction chamber without having fully passed through the gas preheater, and
it supports the load of substrates through the gas distribution plate and load supporting plate and transfers the weight to the susceptor bottom wall without the need for a separate supporting structure for the load supporting plate.
The above contributes to the efficiency of the gas preheating and compactness of the structure located at the bottom of the furnace.
The provision of flow adjusting nozzles which may be inserted in the passages of the gas distribution plate, makes it possible to feed stacks of substrates with a larger flow of reactive gas compared to other stacks of substrates. It is thus possible to compensate for a gradient of temperature between different stacks of substrates in order to achieve a substantially uniform densification. Indeed, the deposition rate of the matrix material varies as a function of the temperature and of the flow of reactive gas.
According to a particular aspect of the invention, the heat exchange assembly of the gas preheater comprises a plurality of spaced apart plates surrounded by the sleeve and extending substantially horizontally between the susceptor bottom wall and the gas distribution plate, the plates of the heat exchange assembly being made of a heat conductive foil material. The use of foil material such as graphite foil material or of C/C composite material makes it possible to reduce the thickness of the plates, hence the bulk of the gas preheater. The plates, which may be of a substantially circular form, are then preferably spaced apart by means of radially extending spacers interposed therebetween.
According to another particular aspect of the invention, the plates of the heat exchange assembly include at least one pair of plates located one immediately above the other in which one plate has perforations only in a central part thereof and the other plate has perforations only in the peripheral part thereof. Thus, the gas is forced to follow a tortuous path, whereby an efficient preheating may be achieved within a limited volume. The gas distribution plate and the load supporting plate may be formed of one and same plate, or of two different plates located one above the other. In the latter case, a plurality of ducts are provided each for connecting a passage of the gas distribution plate to a corresponding passage of the load supporting plate. Each duct may be provided with an insert made of a heat conductive material for achieving heat exchange with reactive gas flowing in the duct and thus completing preheating of the gas.
According to a further aspect of the invention, a process is provided for controlling distribution of preheated reactive gas in a CVI furnace for densification of annular porous substrate loaded in a reaction chamber of the furnace in a plurality of vertical stacks, each stack comprising superposed substrates defining an internal volume of the stack, the reaction chamber being heated by a susceptor having an internal wall delimiting the reaction chamber,
said process comprising admitting the reactive gas into a preheating zone at the bottom of the furnace, preheating the reactive gas by passing it through the preheating zone, dividing the preheated reactive gas into a plurality of separate flows at respective outlets of the preheating zone, and directing the separate flows of reactive gas into respective internal volumes of the stacks of annular substrates,
wherein the separate flows of reactive gas are adjusted as a function of the location of the corresponding stacks of substrates within the reaction chamber.
Preferably, the separate flow of reactive gas directed into the internal volume of a stack of substrates located farther from the internal wall of the susceptor than another stack of substrates is larger than the separate flow of gas directed into the internal volume of said another stack of substrates.
The separate flows of reactive gas may be adjusted by inserting nozzles having different cross-sections into passages formed in a gas-distribution plate covering a gas preheating chamber in the gas preheating zone.